Color-plus-clear coating systems involving the application of a colored or pigmented base coat to a substrate followed by the application of a transparent or clear topcoat to the base coat have become very popular as original finishes for automobiles. The color-plus-clear systems have outstanding gloss and distinctness of image. The clear coat is particularly important for these properties.
Topcoat film-forming compositions, particularly those used to form the transparent clear coat in color-plus-clear coating systems for automotive applications, are subject to defects that occur during the assembly process as well as damage from numerous environmental elements. Such defects during the assembly process include paint defects in the application or curing of the base coat or the clear coat. Damaging environmental elements include acidic precipitation, exposure to ultraviolet radiation from sunlight, high relative humidity and high temperatures, defects due to contact with objects causing scratching of the coated surface, and defects due to impact with small, hard objects resulting in chipping of the coating surface.
Typically, a harder more highly crosslinked film may exhibit improved scratch resistance, but it is less flexible and much more susceptible to chipping and/or thermal cracking due to embrittlement of the film resulting from a high crosslink density. A softer, less crosslinked film, while not prone to chipping or thermal cracking, is susceptible to scratching, waterspotting, and acid etch due to a low crosslink density of the cured film.
Further, elastomeric automotive parts and accessories, for example elastomeric bumpers and hoods, are typically coated “off site” and shipped to automobile assembly plants. The coating compositions applied to such elastomeric substrates are typically formulated to be very flexible so the coating can bend or flex with the substrate without cracking. To achieve the requisite flexibility, coating compositions for use on elastomeric substrates often are formulated to produce coatings with lower crosslink densities or to include flexibilizing adjuvants which act to lower the overall film glass transition temperature (Tg). While acceptable flexibility properties can be achieved with these formulating techniques, they also can result in softer films that are susceptible to scratching. Consequently, great expense and care must be taken to package the coated parts to prevent scratching of the coated surfaces during shipping to automobile assembly plants.
Prior art attempts to improve mar resistance have also included the addition of micron- and submicron-sized particles such as colloidal silica to coating compositions. Such particles typically have highly active (and reactive) surfaces, often due to surface treatments, and as a result the particles tend to agglomerate during production thereof or during incorporation into the coating composition. Agglomeration of the particles prevents high loading of the particles into a coating composition because the viscosity of the composition increases unacceptably. Additionally, agglomeration of the particles may affect the optical properties of the coating, reducing the gloss and clarity thereof because of light scattering.
A number of patents teach the use of a coating comprising a dispersion of colloidal silica in an alcohol-water solution of a partial condensate of a silanol of the formula RSi(OH)3 wherein at least 70 weight percent of the partial condensate is the partial condensate of CH3Si(OH)3. Representative, nonlimiting examples are U.S. Pat. Nos. 3,986,997, 4,027,073, 4,239,738, 4,310,600 and 4,410,594.
U.S. Pat. No. 4,822,828 teaches the use of a vinyl functional silane in an aqueous, radiation curable, coating composition which comprises: (a) from 50 to 85 percent, based on the total weight of the dispersion, of a vinyl functional silane, (b) from 15 to 50 percent, based on the total weight of the dispersion of a multifunctional acrylate, and (c) optionally, from 1 to 3 weight percent of a photoinitiator. The vinyl-functional silane is the partial condensate of silica and a silane, such that at least sixty percent of the silane is a vinyl-functional silane conforming to the formula (R)aSi(R′)b(R″)c wherein R is allyl or vinyl functional alkyl; R′ is hydrolyzable alkoxy or methoxy; R″ is non-hydrolyzable, saturated alkyl, phenyl, or siloxy, such that a+b+c=4; and a≧1; b≧1; c≧0. The patent discloses that these coating compositions may be applied to plastic materials and cured by exposure to ultraviolet or electron beam irradiation to form a substantially clear, abrasion resistant layer.
Similarly, U.S. Pat. No. 5,914,162 teaches the use of colloidal inorganic oxide particles in a radiation-curable coating composition comprising non-silyl polyethylenically unsaturated monomers and oligomers. The coatings exhibit abrasion resistance.
U.S. Pat. No. 5,686,012 describes modified particles comprising inorganic colored and/or magnetic particles as core particles, and at least one polysiloxane modified with at least one organic group which is coated on the surfaces of the core particles. The patent also discloses a water-based paint comprising a paint base material and the modified particles as the pigment as well as a process for producing the modified particles.
U.S. Pat. No. Reissue 30,450 discloses finely divided particulate inorganic pigments surface modified with an amino organosilane. Thermosetting resins incorporating such pigments exhibit improved physical properties, such as abrasion resistance.
U.S. Pat. No. 5,853,809 discloses clear coats in color-plus-clear systems which have improved scratch resistance due to the inclusion in the coating composition of inorganic particles such as colloidal silicas which have been surface modified with a reactive coupling agent via covalent bonding.
Despite recent improvements in color-plus-clear coating systems, there remains a need in the automotive coatings art for topcoats having good initial scratch resistance as well as enhanced retained scratch resistance without embrittlement of the film due to high crosslink density.